The alpha2 macroglobulins (alpha M) are plasma proteins present in all classes of vertebrates which function as broad-spectrum proteinase inhibitors and as elements in a unique system for clearance of active endopeptidases from the circulation. We have recently identified a proteinase inhibitory activity in the plasma of an arthropod, the American horseshoe crab, Limulus polyphemus (Quigley & Armstrong 1983 a,b,c). The present proposal seeks to investigate the possibility that the Limulus inhibitor is homologous in structure and function to vertebrate alpha M. If such a homology can be demonstrated, then alpha M is presumably of sufficient functional importance to have been preserved over the estimated 0.55 - 0.75 billion years of evolution separating the ancestral stocks of vertebrates and arthropods. Limulus should prove to be a useful model organism to study the function of the alpha M-like inhibitor because the animal is abundant, contains a large quantity of easily obtainable plasma, and the alpha M-like inhibitor is the only proteinase inhibitor in the plasma (in contrast to vertebrates, which possess a variety of plasma proteinase inhibitors). The latter aspect is significant since the presence of the other proteinase inhibitors in mammalian blood has complicated the elucidation of the exact physiological roles and functions of alpha M. We propose to take advantage of this attributed and our recent establishment of a purification scheme for Limulus alpha M (Quigley & Armstrong, 1985) to examine its role in the clearance of endopeptidases from the circulation and the possible existence of specific alpha M- proteinase receptors in Limulus blood cells. In addition we plan to study the reasons for the apparent inability of Limulus alpha M to inhibit the key proteinase in the blood coagulation cascade in Limulus. The Limulus clotting system has been suggested to represent a primitive model for the evolutionary precursors of vertebrate clotting systems. A variety of other invertebrates will be examined for the presence of alpha M homologues in the plasma.